Incorporation of catalytic dehydrogenation into fischer-tropsch synthesis to significantly reduce carbon dioxide emissions

Abstract

A new method of producing liquid transportation fuels from coal and other hydrocarbons that significantly reduces carbon dioxide emissions by combining Fischer-Tropsch synthesis with catalytic dehydrogenation is claimed. Catalytic dehydrogenation (CDH) of the gaseous products (C1-C4) of Fischer-Tropsch synthesis (FTS) can produce large quantities of hydrogen while converting the carbon to multi-walled carbon nanotubes (MWCNT). Incorporation of CDH into a FTS-CDH plant converting coal to liquid fuels can eliminate all or most of the CO 2 emissions from the water-gas shift (WGS) reaction that is currently used to elevate the H 2 level of coal-derived syngas for FTS. Additionally, the FTS-CDH process saves large amounts of water used by the WGS reaction and produces a valuable by-product, MWCNT.

1. A method of producing liquid fuels from coal or coal+biomass or other solid hydrocarbons, comprising:
 Gasifying a starting material selected from a group consisting of coal, biomass, waste hydrocarbons, and mixtures thereof to produce a syngas; 
 Subjecting said syngas to Fischer-Tropsch synthesis (FTS) to produce a hydrocarbon product stream; 
 Separating said hydrocarbon product stream into C1-C4 hydrocarbons and C5+hydrocarbons that are used as liquid fuels; 
 Subjecting said C1-C4 hydrocarbons to catalytic dehydrogenation (CDH) in the presence of a catalyst to produce hydrogen (H 2 ) and multi-walled carbon nanotubes (MWCNT); 
 Utilizing energy released in cooling the syngas from a gasification temperature of between about 1,500-1,600° K to a useful operating temperature for the CDH reaction of about 1,000° K to provide the activation enthalpy required for catalytic dehydrogenation; 
 Mixing said H 2  with the syngas from said gasifier to increase the H 2 /CO ratio of the syngas for FTS to 2.0 or higher thereby enabling production of liquid fuels by combined FTS and CDH with significantly reduced CO 2  emissions relative to conventional FTS processing; 
 Cleaning said MWCNT and dissolving said CDH catalyst in a cleaning solution; 
 Recovering the MWCNT from the cleaning solution by centrifugation, filtration, or other techniques; 
 Recovering said catalyst from the resulting cleaning solution by standard precipitation methods and recycling it to the CDH reactor. 
 
     
     
       2. The method of  claim 1  including using dilute nitric acid as said cleaning solution, recovering said catalyst from said cleaning solution, and recycling said catalyst. 
     
     
       3. The method of  claim 1  including using a starting material comprising between about 50 and 100 weight percent coal, between about 0 and about 50 weight percent biomass and/or between about 0 and about 50 weight percent of waste hydrocarbons, including plastics and rubber products. 
     
     
       4. The method of  claim 1  including selecting said biomass from a group of materials consisting of switchgrass, corn stover, agricultural wastes, lawn wastes, woody biomass from lumbering wastes, and waste products from paper production. 
     
     
       5. The method of  claim 1 , including subjecting said syngas to FTS at temperatures in the approximate range of 220-300° C. in the presence of a Co-based FTS catalyst. 
     
     
       6. The method of  claim 1 , wherein said separation is accomplished by condensation. 
     
     
       7. The method of  claim 1 , wherein said catalytic dehydrogenation includes passing said (C1-C4) hydrocarbons over a catalyst comprising a binary metallic iron-based alloy, Fe-M, where the secondary metal M may be selected from a group consisting of Ni, Mo, Mn, Pd, V, Cr, Co, Zn, W and any mixtures thereof. 
     
     
       8. The method of  claim 7 , including using an Fe/M alloy ratio in a range of 1-4. 
     
     
       9. The method of  claim 8  including depositing said Fe/M alloy on a support. 
     
     
       10. The method of  claim 9  including selecting said support from a material selected from a group consisting of Al 2 O 3 , SiO 2 , Mg 5 AlO, MgO, and multi-walled carbon nanotubes (MWCNT). 
     
     
       11. The method of  claim 10 , including reducing said Fe/M catalysts to their active metallic state, either in hydrogen or syngas; an austenitic metallic alloy is preferred but other metallic structures can also achieve catalytic dehydrogenation. 
     
     
       12. A liquid fuel production facility, comprising:
 a gasification unit to produce a syngas from a starting material selected from a group consisting of coal, biomass, waste hydrocarbons, waste carbon and mixtures thereof; 
 a Fischer-Tropsch unit downstream from said gasification unit to produce a hydrocarbon product stream from said syngas, said hydrocarbon product stream including C1-C4 hydrocarbons and C5+ hydrocarbons used as liquid fuels; 
 a separation unit between said Fischer-Tropsch unit and said catalytic dehydrogenation unit to separate said hydrocarbon product stream into C1-C4 hydrocarbons and C 5   +  hydrocarbons used as liquid fuels; 
 a catalytic dehydrogenation unit downstream from said separator unit to produce hydrogen gas and carbon nanotubes from said C1-C4 hydrocarbons; 
 a mixing unit downstream from the gasifier and the catalytic dehydrogenation unit which uniformly mixes hydrogen from the CDH unit and syngas from the gasification unit to produce modified syngas with significantly enhanced [H 2 ]/[CO] ratios; and 
 a MWCNT cleaning and catalyst recovery unit.